Add option to preserve property name casing in OpenAPI schema generation

[Darka7]

Título: Add option to preserve property name casing in OpenAPI schema generation

Issue:
Currently OpenAPI schema generation converts property names to camelCase by default. There's no built-in way to preserve original property casing from C# classes.

Request:
Add configuration option to preserve original property casing when generating OpenAPI schemas, similar to how JsonSerializerOptions.PropertyNamingPolicy = null works for JSON serialization.

Example:

// Current output for class property "UserName"
// OpenAPI schema shows "userName"

// Desired configuration
builder.Services.AddOpenApi(options => 
{
    options.PreservePropertyCasing = true; // Proposed option
}); 

Use case:
Maintaining consistent property casing between C# models and API documentation, especially important when working with systems that expect exact property name matches.

[martincostello]

OpenAPI uses the same JSON serialization options that Minimal APIs uses. If you're using MVC you'll need to configure the two sets of JSON options to match each other. There's no need to add additional options specific to OpenAPI.

Or, explicitly annotate your models with JSON serialization attributes so they are always serialised the same way regardless of the settings.

[Darka7]

Solution to preserve property names in .NET OpenAPI and JSON

The OpenAPI schema and JSON responses were automatically converting property names to camelCase. The solution is to configure both the MVC and HTTP JSON options globally:

builder.Services.Configure<Microsoft.AspNetCore.Mvc.JsonOptions>(option =>
{
    option.JsonSerializerOptions.DictionaryKeyPolicy = null;
    option.JsonSerializerOptions.PropertyNamingPolicy = null;
    option.JsonSerializerOptions.Converters.Add(new DateJsonConverter());
    option.JsonSerializerOptions.Converters.Add(new DateOnlyJsonConverter());
    if (AddConverters != null) AddConverters(option,null);
});

builder.Services.Configure<Microsoft.AspNetCore.Http.Json.JsonOptions>(option =>
{
    option.SerializerOptions.PropertyNamingPolicy = null;
    option.SerializerOptions.PropertyNamingPolicy = null;
    option.SerializerOptions.Converters.Add(new DateJsonConverter());
    option.SerializerOptions.Converters.Add(new DateOnlyJsonConverter());
    if (AddConverters != null) AddConverters(null,option);
});

Thanks to the .NET team for clarifying that OpenAPI uses the same JSON serialization settings!

Hope this helps others with the same issue! 👍